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New Protocol
STATUS: This is currently a proposed draft as of October 25, 2008 Drizzle Protocol ---------------- The Drizzle protocol works over TCP, UDP, and Unix Domain Sockets (UDS, also known as IPC sockets), although there are limitations when using UDP (this is discussed below). In the case of TCP and UDS, a connection is made, a command is sent, and a response loop is started. Socket communication ends when either side closes the connection or a QUIT command is issued. TCP and UDS communications will be full duplex. This means that as the client is sending a command, it is possible for the server to report an error before the sending of data completes. This allows the server to do preliminary checks (table exists, authentication, ...) before a request is completely sent so the client may abort. This will primarily be used for large requests (INSERTing large BLOBs). TCP and UDS communications will also allow for pipe-lining of requests and concurrent command execution. This means a client does not need to wait for a command to finish before a new command is sent. It is even possible a later command issued will complete and have a result before an earlier command. Result packets may be interleaved so a client issuing concurrent commands must be able to parse results concurrently. UDP sockets are supported to allow small, fast updates for applications such as statistical gathering. Since UDP does not guarantee delivery, this method should not be used for applications that require reliable transport. When using UDP, the authentication packet (if needed) and command packet are bundled into a single UDP packet and sent. This puts a limitation on the size of the request being made, and this limit can be different between network hosts. The absolute limit is 65,507 bytes (28 bytes used for IPv4 and UDP headers), but again, this can depend on the network hosts. Responses are optional when issuing UDP commands, and this preference is specified in the handshake packet. All sizes given throughout this document are in bytes. Byte order for all multi-byte binary objects such as lengths and mutli-byte bit-fields are packed little-endian. Packet Sequence Overview ------------------------ The sequence of packets for a simple connection and command that responds with an OK packet: C: Command S: OK The sequence of packets for a simple connection and query command with results: C: Command S: OK S: Fields (optional, multiple packets) S: Rows (multiple packets) S: EOF When authentication is required for a command, the server will ask for it. For example: C: Command S: Authentication Required C: Authentication Credentials S: OK S: Fields S: Rows S: EOF The server will use the most recent credential information when processing subsequent commands. If a client wishes to multiplex commands on a single connection, it can do so using the command identifiers. Here is an example of how the packets could be ordered, but this will largely depend on the servers ability to process the commands concurrently and the processing time for each command. C: Command (Command ID=1) C: Command (Command ID=2) S: OK (Command ID=2) S: Field (Command ID=2) S: OK (Command ID=1) S: Fields (Command ID=2) S: Rows (Command ID=2) S: EOF As you can see, the commands may be executed with results generated in any order, and the packet containing the results may be interleaved. Length Encoding --------------- Some lengths used within the protocol packets are length encoded. This means the size of the length field will vary between 1 and 9 bytes, and is determined by the value of the first byte. 0-252 - Actual length 253 - NULL value (only applicable in row results) 254 - Following 8 bytes hold length 255 - Depends on context, usually signifies end Packets ------- Packets consist of two layers. The first is meant to be small, simple, and have just enough information for fast router and proxy processing. It consists of a fixed-size part, along with a variable sized client id (explained later), a series of chunked data, followed by a checksum at the end. The chunked transfer encoding allows for not having to pre-compute the packet data length before sending, and support packets of any size. It also allows for a large packet to be aborted gracefully (without having to close the connection) in the event of an error. The first part of a packet is: 1-byte Magic number, the value should be 0x44. 1-byte Protocol version, currently 1. 4-byte OPAQUE. This is a unique number among all other queries currently being executed on the connection. The client is responsible for choosing a unique number while generating a command packet, and all response packets associated with that command must have the same command ID. Once a command has been completed, the client may reuse the ID. 1-byte Command/result code. For commands, this may be: 1 ECHO - The entire packet is simply echoed back to the caller. 2 SET - Set protocol options. 3 QUERY - Execute query. 4 QUERY_RO - Same as QUERY, but hints that this is a read-only query. This is only useful for routers/proxies who may want to redirect the request to a read slave. Result codes may be: 1 OK - Single packet success response. No data associated with the result besides parameters. 2 ERROR - Single packet error response. 3 ROW - Start of a multi-packet result set. 4 END_OF_ROW - End of a multi-packet result set. 2-byte Sharding Key length. X-byte Sharding Key (length is value of client ID length). The client ID is there for the client and routers/proxies to use. The server treats this as opaque data, and will only preserve it to send in responses. This can be used as a sharding key, to keep state information in a proxy, or any other use. Next, zero or more chunks are given, terminated by a chunk length of 0. Each chunk consist of a length and then that amount of data. 2-byte Chunk length X-byte Chunk (length is value of chunk length) After the the chunk length of 0 is given, a checksum value is given that was computed for the entire packet. 4-byte Checksum The second layer of the protocol is encapsulated inside of the chunked encoding. This consists of zero or more packet parameters, an end of parameter marker, followed by an optional data set that is given until the end of a packet (or the end of all chunks). Packet Parameters ----------------- Packet parameter names are defined in a global name-space, although not all parameters are relevant for all packet types. Parameters are enumerated, and the name is specified with a 1-byte value representing the enumerated name. Each packet parameter may have a value associated with it, and each parameter defines the size and how that value is given. The list of possible packet parameters are: 0 END_OF_PARAMETERS - Marks the end of a parameter list. Parameters used for setting options: 1 AUTH - 1-byte value with authentication mechanism to use. Possible values are: 0 - None. 1 - MD5 on user and password. 2 - 3-way handshake. 2 CHECKSUM - 1-byte value with preferred checksum type. Possible values are: 0 - None. 1 - CRC32 3 COMPRESSION - 1-byte value with preferred compression type. Possible values are: 0 - None. 1 - zlib. 2 - bzip2. 4 FIELD_ENCODING - 1-byte value with preferred checksum type. Possible values are: 0 - String. 1 - Native. 5 FIELD_INFO - 1-byte value to determine if field information should be sent. Possible values are: 0 - None. 1 - Send field info. Parameters used in responses: 6 STATUS - 4-byte bit field. 7 ROWS_EFFECTED - Length-encoded count of rows effected. 8 ROWS_SCANNED - Length-encoded count of rows scanned. 9 WARNINGS - Length-encoded count of warnings encountered. 10 INSERT_ID - Last insert ID. 11 ERROR_CODE - 4-byte error code. 12 ERROR_STRING - Length-encoded string. 13 SQL_STATE - Length-encoded string. 14 FIELD_COUNT - 4-byte integer. 15 FIELD_TYPE - 2-byte enumerated type. 16 FIELD_LENGTH - Length-encoded value. 17 FIELD_FLAGS - 4-byte bit-field. 18 DB_NAME - Length-encoded string. 19 TABLE_NAME - Length-encoded string. 20 ORIG_TABLE_NAME - Length-encoded string. 21 FIELD_NAME - Length-encoded string. 22 ORIG_FIELD_NAME - Length-encoded string. 23 DEFAULT_VALUE - Length-encoded string. "Length-encoded string" means a length-encoded value, followed by a string of that length. Command ------- Inside of the chunked data, command packets consist of zero or more parameters depending on which options are being set, followed by a end of parameter marker, and then all data until the end of the chunks are considered arguments for the command. For a QUERY, this will be the actual query to run. OK/ERROR -------- The server responds with an OK or ERROR if no row data is given. A list of parameters may follow, and the marked with an end of parameter value. Row --- A row packet consists of a series of parameters, followed by the end of parameter, and then a series of length-encoded values holding row field data. The FIELD_COUNT parameter must be given before any rows, as this indicates when a start of a new row happens. The field data may either be in string format or native data type, depending on the value of FIELD_ENCODING. You may have multiple rows inside of a single ROW result packet, but all of them do not need to be (and should not be in the case of large result sets). This allows for other result packets to be interleaved even with large result sets.